Bitumens are naturally occurring or pyrolytically obtained substances of dark to black color consisting almost entirely of carbon and hydrogen with very little oxygen, nitrogen, and sulfur. Bitumens vary widely in hardness and volatility, ranging from crude oil to asphaltines.
Asphalt is a naturally occurring bitumen. It is also a petroleum byproduct, from which it is manufactured in commercial quantities by the removal of volatile components. Asphalt is composed of hydrocarbons and heterocyclic compounds having molecular weights varying from about 400 to above 5000. It is both thermoplastic and viscoelastic; i.e., at high temperatures or over long loading times it behaves as a viscous fluid, while at low temperatures or short loading times it behaves as an elastic body.
The three distinct types of asphalt made from petroleum residues are straight-run, air blown, and cracked. Straight run asphalt, characterized by a nearly viscous flow, is used in the construction of pavement surfaces for roads and airport runways. In this application, thermoplastic behavior is an undesirable characteristic in that the high temperatures experienced by roadway surfaces in a warmer climate or on hot days would result in excessive deformation of a thermoplastic asphalt when subjected to normal vehicle or aircraft loads. Ideally, the asphalt used in a pavement surface application would exhibit no flow, either viscous or thermoplastic, yet remain elastic. Airblown asphalt is resilient and has a viscosity that is less susceptible to temperature change than that of straight-run asphalt. It is used mainly for roofing, pipe coating, paints, underbody coatings, and paper laminates. This type of asphalt, while suitable for insulation and prophylactic applications, is not of sufficient hardness for pavement surfaces. Cracked asphalt, with limited applications such as dust laying or as an insulation board saturant, has a nearly viscous flow, but its viscosity is more susceptible to temperature change than straight-run asphalt.
Tannin or a tannin compound in combination with a surfactant and a heated bitumen to provide a bitumen-in-water emulsion have been described in the art. The published Kao Soap Company Japanese patent document by Ryooichi, et al., Japanese patent document 63-17960, reports a slow-setting, cationic, asphalt-in-water emulsion made from an emulsifying composition, including a surfactant, a tannin or tannic acid compound, calcium chloride and hydrochloric acid to adjust the pH. Quebracho is listed as one of the tannic acid compounds that can be used as a tannin compound. The emulsifying composition is combined with heated asphalt in the ratio of 40% w/w emulsifying agent to 60% w/w asphalt. However, attempts to reproduce the reported results of the Kao emulsion have been unsuccessful. The art is silent as to the mixing of quebracho with a bitumen to form a modified bitumen composition.
Quebracho is a natural product extracted from the heartwood of the Schinopsis trees that grow in Argentina and Paraguay. Quebracho is a well characterized polyphenolic and is readily extracted from the wood by hot water. Quebracho is currently widely used as a tanning agent. It is also used as a mineral dressing, as a dispersant in drilling muds, and in wood glues.
Quebracho is commercially available as a crude hot water extract, either in lump, ground, or spray-dried form, or as a bisulfite treated (refined) spray-dried product that is completely soluble in cold water. Quebracho is also available in a "bleached" form which can be used in applications where the dark color of unbleached quebracho is undesirable. The production of quebracho has been carried out for many years. Although quebracho is a natural product and has some variation in composition, it is a consistent source of the polyphenols that we have found are useful as a bitumen modifier for the modified bitumen compositions of this invention.
A road pavement's ability to support loads depends primarily upon the magnitude of the load, how often it is applied, the supporting power of the soil underneath, and the type and thickness of the pavement structure. Rigid pavements are constructed from concrete. Flexible pavements have less bending resistance than rigid pavements, and are typically composed of aggregate (sand, gravel, or crushed stone) and bituminous material as the top layer (approximately 2 inches), and an overlying seal coat. As noted above, the asphalt used in the construction of pavement surfaces preferably exhibits minimum thermoplastic flow, has a high viscosity (minimum viscous flow), and high hardness. Unmodified or base asphalt products currently available do not have this ideal set of characteristics.
Consequently, there is a long felt need for a bituminous or asphalt composition having higher viscosity, greater hardness and less thermoplasticity as compared to an unmodified or base asphalt, and particularly for a bitumen or asphalt composition suitable for use as a pavement layer in roadway or airport runway construction, as well as other applications where higher viscosity, low thermoplasticity and greater hardness are desired characteristics.
The strength of asphalt/concrete pavings will be adversely affected if moisture penetrates the asphalt-aggregate interface. The presence of water in the interface between the asphalt and the surface of the aggregate weakens the bond between the aggregate and the asphalt. Consequently, the asphalt is "stripped" away from the aggregate with resulting degradation of tensile strength of the asphalt/aggregate mixture.
Stripping is the breaking of the adhesive bond between the aggregate surface and the asphalt cement. Usually stripping begins at the bottom of the asphalt layer and moves upward until the pavement structure is weakened. Under traffic, cracks appear and, in advanced stages, the pavement begins to disintegrate; and, always, water is present. In one way or another the water gets between the asphalt film and the aggregate surface and, because the aggregate surface has a greater affinity for water than for asphalt, the adhesive bond is broken.
There is only one cause of stripping--water getting between an asphalt film and an aggregate surface and replacing the asphalt as the aggregate's coating. The water may reach the pavement structure is several ways. Among them are water in or on improperly dried aggregates; rainfall seeping through shoulders, cracks, or porous pavement, subsurface water from higher ground producing a hydrostatic head, capillary water from the subgrade, and water vapor from the subgrade. Once in place, the water may get to the aggregate in a number of different ways. The age and type of aggregate can affect the stripping resistance of the overlying asphalt. The age of the aggregate is important in that it is known that newly crushed aggregate has poor stripping resistance. If such an aggregate is used to soon after it is crushed, stripping may occur. Ostensibly the surface energy of the fresh aggregate surface is such that the surface has a much higher affinity for water than for asphalt.
The type of aggregate is known to affect the stripping resistance of asphalt to water. It is thought that aggregates with a high silica content, sometimes called "hydrophillic" (water loving), are apt to strip and that aggregates with low or no silica content, sometimes called "hydrophobic" (water hating), are not. Practically, however, few aggregates show complete resistance to the action of water under all conditions of use. As a rule, though, it is safer to use hydrophobic carbonate rock, such as limestone, if it is available. Regardless of the precautions taken, water will eventually find its way into the aggregate/asphalt mixture.
Anti-stripping additives are often used in the preparation of the asphalt concrete paving mixtures to improve the resistance of the paving mixtures to the action of water. Such anti-stripping agents may be certain amines that are added to the hot asphalt or they may be ground solids, such as hydrated lime or portland cement, that are mixed with the aggregate prior to mixing with the asphalt. The performance, or effectiveness, of an anti-stripping agent is determined by comparing the tensile strength of two sets of laboratory compacted pavement mixtures; one set kept dry and the other set saturated and water conditioned. The performance of the anti-stripping agent is expressed as a percentage of the tensile strength of the wet set as compared to the dry set (as described in the American Society for Testing Materials, test procedure D 4867-92).
The chemicals most often used as asphalt/coal tar anti-stripping agents are long chain organic compounds containing one or more amine moieties. In theory, the long organic chain, being soluble in asphalt, is anchored in the asphalt, whereas the positively charged amine nitrogen has an affinity for the aggregates. Since the asphalt is firmly attached to the long chain organic "tail" of the anti-stripping agent and the aggregate is firmly bonded to the amine "head" of the anti-stripping agent, the presence of water is less likely to cause the asphalt to strip from the aggregate.
Anti-stripping agents may be added directly to the asphalt or it may be premixed with the aggregate prior to mixing with asphalt. When an amine anti-stripping agent is added directly to the asphalt, only a small fraction of the agent is actually used. The time available for the amine to migrate through the asphalt and to the aggregate surface is limited; only a small portion reaches the aggregate before the asphalt cools and no further migration is possible.
Further, the anti-stripping agent must be thermally stable if it is to be mixed directly with the asphalt, since long delays (for example, due to rainy weather) may result in the asphalt being maintained at high temperature for several days. Consequently, to maintain thermal stability as an anti-stripper, any active amine hydrogens should be replaced with other, less active groups, otherwise the amine will lose its anti-stripping effectiveness if the asphalt is stored at high temperatures for any appreciable time.
For these reasons, it can be more efficient to apply the anti-stripping agent directly to the aggregate surface, as is done with hydrated lime or portland cement in a separate mixing step, prior to mixing with asphalt.
Currently, commercially available anti-stripping agents have a long chain organic molecule with one or more amine moieties making them difficult to work with. The long chain organic portion is characterized by relatively high molecular weights and diffuse charge distribution resulting in semi-solid or highly viscous materials that must be heated before they can be readily metered into hot asphalt.
Further, most of these anti-stripping agents are not water soluble. Consequently, prior to applying (e.g., spraying) the agent directly on the aggregate prior to mixing with asphalt, they must be dissolved in an organic solvent. However, the use of an organic solvent would result in undesirable organic emissions, potentially triggering an environmental permit requirement and/or hazardous materials and/or hazardous waste regulations. Further, any residual solvent presents the risk of inclusion or entrainment of an undesirable high boiling organic contaminant into the pavement mixture. Hence, the currently available organo-amine anti-stripping agents are not convenient in their use and do not exhibit those attributes that are desirable in a universal anti-stripping additive.
Accordingly, there is a long felt need for an anti-stripping additive that may be applied either directly to the aggregate as an aqueous solution, or mixed directly with the asphalt, that is thermally stable, that does not risk contamination of the asphalt with a high boiling solvent, and is innocuous in its use and disposal.